*Purpose: 1) To describe prevalence of overweight and obesity and changes in body mass index (BMI) and height Z-score at transplant time (T0) and 12 months after transplantation (T1) in pediatric kidney transplant recipients. 2) To identify factors associated with weight excess at 12 months 3) To assess patient and graft survival in children with and without overweight or obesity.

*Methods: A retrospective cohort study was conducted including consecutive pediatric recipients with a 1st KTx performed at Hospital de Pediatria JP Garrahan between January 1, 2013 and October 31, 2017 with an outpatient follow-up for at least 12 months (n = 168 patients). Patients were excluded if they had a 2^nd and or combined Tx (n=7), lost their graft (n=4), or had missing anthropometric data in the medical records (n=24). Change in BMI was defined as the gain in BMI ≥1 SD and catch-up growth an increase H/A Z-score ≥ 0.5 SD. To analyze factors associated with weight excess at 12 months, the dependent variable was prevalence of overweight (BMI = weight in kilograms/height in m²) ≥ 85^th pc and < 97^th pc, or obesity (BMI ≥ 97^th pc) according to the WHO reference population and/or a change in BMI. In children with H/A Z-score < -2, BMI was expressed relative to height/age; i.e. age at which the child’s height would be at the 50^th pc. Increase in H/A Z-score ≥ 0.5 SD between KTx and 12 months after was considered catch-up growth. Median follow-up was 26.4 months (IQR: 18.6-43.8).

*Results: We included 133 patients with 12 ± 4 years (r: 3.4-17.7) of age at KTx. At T0 93% (n=124) had a normal BMI, 5.3% (n = 7), had overweight and obesity (OW-OB) and 1 patient was underweight; and at T1 13.8% (n = 17) were OW-OB (p = 0.029). After KTx, 123 patients (92%) had an increase in their BMI ≥1 SD; median weight gain was 6.1 kg (IQR: 3.5-9.6). Mean H/A Z-score was -2.81±0.18 at T₀ and -2.57±0.18 at T₁ (p = 0.36). Median height gain was 4 cm (IQR: 2-7); 27 patients (20.5%) had catch-up growth and 49 (37%) increased their BMI ≥1 SD. Of them, in only 9 (10%) this change in BMI was associated with height gain. Mean age at T₀ was 10.7±0.7 years and 12.3±0.4 in those with and without catch- up growth, respectively (p=0.07). Comparing patients with and without change in BMI, no differences were found between male vs female recipients (73% vs 26%, p = 0.12), age at Ktx ≤ 5 years (33% vs 37%, p = 0.8), being a deceased vs living-donor receptor (38% vs 61%, p = 0.24), accumulative steroids dose (71.4 ± 3.5 vs 78.3 ± 4 mg/kg, p = 0.22), “steroid pulses” (5% vs 4%, p = 0.9), previous chronic peritoneal dialysis (33% vs 27%, p = 0.66), preemptive KTx (10.6% vs 0%, p = 0.16), post-KTx diabetes ( 04.8%; p=0.11), eGFR at 12m (65.9 ± 2.4 vs 70 ± 3.1 ​​ml / min; p = 0.24), and DGF (13% vs 11%, p = 0.85). Patient survival in those with and without OW-OB was 100% at 4 years. Graft survival at 1, 2, 3 and 4 years was 94% in those with OW-OB and 100%, 97.5%, 93% and 90% in those without OW-OB (p= 0.96).

*Conclusions: Prevalence of overweight-obesity significantly increased at 1 year post-KTx. Change in body weight was the cause of BMI change in 79.5% of KTx children. No variables were associated with change in BMI at 1 year, No differences in patient or graft survival were seen between those with OW-OB vs those with normal weigth or underweight.

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